Looking at the application log in the Exchange server showed an Event ID 403 with source MSExchange Common:

The Certificate named xxxxxxx in the Federation Trust 'Microsoft Federation Gateway' is expired. Please review the Federation Trust properties and the certificates installed in the certificate store of the server.

After checking of course, the Federation certificate was just created… and is indeed valid…..

All that was required was a quick “restart” to the application pools on the server, I usually just restart the MSExchangeServiceHost and MSExchangeProtectedServiceHost services. after that the wizard completed successfully 🙂